After fabrication of circuits on a semiconductor die, detection of faults is often desirable. Detecting these faults without destroying the circuitry on the semiconductor is also useful. Because the circuit was preserved during fault detection, the circuit can also undergo other testing prior to deprocessing. Once the position of the fault is detected and other testing completed, the circuit may be deprocessed in order to determine the exact nature of the fault or to further investigate the properties of the circuit.
Conventional semiconductor dies are mounted on a substrate with the circuitry at the top and the bottom of the die in contact with the substrate. This combination is known as a conventional package. Leads typically connect portions of the circuit at the top of the die to the substrate. In order to detect faults which cause hot spots on such a circuit, a liquid crystal is conventionally used. A thin layer of liquid crystal is poured over the circuits at the top of the die. When power is connected to the circuits, the liquid crystal changes phase over any hot spots in the circuits. Consequently, the liquid crystal changes color over these hot spots in the circuit. These hot spots and, therefore, areas where the liquid crystal has changed color mark the position of certain faults in the circuits. Consequently, the position of faults in the circuit can be detected.
Although conventional liquid crystal detection functions for circuits in conventional packages, this method does not function for some newer semiconductor circuit packages. Circuits for some high speed applications, such as certain microprocessors, cannot function adequately if mounted in a conventional manner. In order to make electrical connection between the circuit on the semiconductor die and the substrate, therefore, the semiconductor die is flipped. Thus, the top of the die, where the circuits are formed, is in contact with the substrate. Separate leads are not used. The combination of substrate and the die having it top in contact with the substrate is known as a flip-chip package.
It is still desirable to detect faults in the circuits of a flip-chip package. However, because the die is flipped, the circuits are not readily accessible. In addition, the portion of the semiconductor die below the circuits is significantly thicker than the portion of the semiconductor die in which the circuits are formed. Consequently, applying the liquid crystal to the back of the die which is readily accessible will not detect faults causing hot spots within the circuits in the flip-chip package.
Accordingly, what is needed is a system and method for detecting faults in circuits on flip-chip packages without destroying the circuits . The present invention addresses such a need.